The use of rotors with foils for cleaning pulp screens by generating pressure pulses as the foil is moved past the screen is a well-known and common technique that has been practiced in the industry for many years. The pressure pulse, specifically the negative pulse, clears the apertures by causing a flow reversal that backflushes the fibres in the apertures. This cleaning technique is reasonably effective, but the maximum negative pressure pulses that conventional foils or rotors can generate effectively are limited. Some specific examples of those found in the art are described below.
PCT application—PCT/SE89/00568 WO 90/05807 published May 31, 1990 inventor Lundberg et al. discloses a typical screening apparatus and teaches the use of wing elements on the rotor (as opposed to foils) constructed so that the leading end of the wing in the direction of rotation is spaced closer to the screen than the trailing end and the wing has a dimension measured in the direction of movement (circumferential direction) that is at least twice the radial dimension of the screen to generate a suction force to draw liquid that has already passed through the screed to the outlet side back through the screen to the inlet side to dilute the pulp on the inlet side and to clean the pores of the screen.
PCT application no PCT/FI/00151—WO 93/22494 published Nov. 11, 1993 to Alajaaski et al. describes a special pulse generator that tends to locally confine the pulse to thereby improve the cleaning operation of the pulse generator which in turn increases screening efficiency
PCT application PCT/US94/04582—WO 94/25183 published Nov. 10, 1994 inventor Egan et al. describes the use of a special adjustable hydrofoil having a moveable section projecting out from it's cambered surface. The position of this moveable section is adjusted to obtain the optimum spacing between the screen and rotor to thereby improve the operation of the screening device.
EP 0950 754 A1 published Oct. 20, 1998 by Alkawa describes a stirring device in the form of a foil that applies fluid pressure against the screen adjacent to the leading end of the foil and a negative pressure for cleaning the screen adjacent to the trailing end of the foil.
U.S. Pat. No. 5,799,798 issued Sep. 1, 1998 to Chen teaches the use of conventional stirrers or foil and uses specially designed screen bars to improve the operation of the screening system.
Japanese patent 93-243392 shows the use of angular bars on the low-pressure side of the screen to improve the operation of the screening device.
In the aircraft industry higher angle of attacks are achieved without separation of the air passing along the foil from the camber surface of the foil by employing cambered airfoils with multi-element configurations. This results in being able to attain higher lift forces by using multi-element airfoils which in effect delay the onset of flow separation from the foil (stall) and allow higher angles of attack and increased camber. The stall condition is delayed by allowing air from the high-pressure side of the wing or foil to pass into the boundary layer of the low-pressure side of the wing. This injection of air re-energizes the boundary layer enabling the flow to remain attached to the foil. Multi-element airfoils are commonly used in aerodynamic applications.